Support user-interactive authentication api in /login (SYN-696)

[matrixbot]

Currently /login uses a stripped-down form of the user-interactive authentication API, where the POSTed parameters look like the contents of an auth dict, and it supports a GET version which replies with a 200 whose contents look like those of the 401 response in the user-interactive auth api.

This is inconsistent and confusing; we should switch it over to the user-interactive authentication API proper (and then spec it so that clients can use it).

(Imported from https://matrix.org/jira/browse/SYN-696)

(Reported by @richvdh)

[matrixbot]

Jira watchers: @richvdh

[matrixbot]

Links exported from Jira:

relates to SPEC-403
relates to SPEC-275
relates to SYN-396

[reivilibre]

I don't see this being fixed given the solution is to replace /login with OIDC and do away with user-interactive auth.